1. Field of the Invention
The present disclosure relates to ultrasonic imaging. More particularly, the invention relates to a system and method that improves contrast agent imaging diagnostic evaluations.
2. Discussion of the Related Art
Ultrasonic imaging has quickly replaced conventional X-rays in many clinical applications because of its image quality, safety, and low cost. Ultrasonic images are typically formed through the use of phased or linear array transducers which are capable of transmitting and receiving pressure waves directed into a medium such as the human body. Such transducers normally comprise multi-element piezoelectric materials, which vibrate in response to an applied voltage to produce the desired pressure waves. Piezoelectric transducer elements are typically constructed of lead zirconate titanate (PZT), with a plurality of elements being arranged to form a transducer assembly. A new generation ultrasonic transducer known as a micro-machined ultrasonic transducer (MUT) is also available. MUTs are typically fabricated using semiconductor-manufacturing techniques with a number of elements typically formed on a common substrate to form a transducer assembly. Regardless of the type of transducer element, the transducer elements may be further assembled into a housing possibly containing control electronics, the combination of which forms an ultrasonic probe. The ultrasonic probe may include acoustic matching layers between the surface of the various types of elements and the probe body. Ultrasonic probes may then be used along with an ultrasonic transceiver to transmit and receive ultrasonic pressure waves through the various tissues of the body. The various ultrasonic responses may be further processed by an ultrasonic imaging system to display the various structures and tissues of the body.
To obtain high quality images, the ultrasonic probe must be constructed so as to produce specified frequencies of pressure waves. Generally speaking, low frequency pressure waves provide deep penetration into the medium (e.g., the body), but produce poor resolution images due to the length of the transmitted wavelengths. On the other hand, high frequency pressure waves provide high resolution, but with poor penetration. Accordingly, the selection of a transmitting frequency has involved balancing resolution and penetration concerns. Unfortunately, resolution has suffered at the expense of deeper penetration and vice versa. Traditionally, the frequency selection problem has been addressed by selecting the highest imaging frequency (i.e., best resolution) which offers adequate penetration for a given application. For example, in adult cardiac imaging, frequencies in the 2 MHz to 3 MHz range are typically selected in order to penetrate the chest wall. Lower frequencies have not been used due to the lack of sufficient image resolution. Higher frequencies are often used for radiology and vascular applications where fine resolution is required to image small lesions and arteries affected by stenotic obstructions.
Recently, new methods have been studied in an effort to obtain both high resolution and deep penetration. One such method is known as harmonic imaging. Harmonic imaging is grounded on the phenomenon that objects, such as human tissues, develop and return their own non-fundamental frequencies, i.e., harmonics of the fundamental frequency. This phenomenon and increased image processing capabilities of digital technology, make it is possible to excite an object to be imaged by transmitting at a low (and therefore deeply penetrating) fundamental frequency (fo) and receiving reflections at a higher frequency harmonic (e.g., 2fo) to form a high resolution image of an object. By way of example, a wave having a frequency less than 2 MHz can be transmitted into the human body and one or more harmonic waves having frequencies greater than 3 MHz can be received to form the image. By imaging in this manner, deep penetration can be achieved without a concomitant loss of image resolution.
However, in order to achieve the benefits of transmitting at a lower frequency for tissue penetration and receiving a harmonic frequency for improved imaging resolution, broadband transducers are required which can transmit sufficient bandwidth about the fundamental frequency and receive sufficient bandwidth about the harmonic(s). The s4 transducer available with the SONOS(trademark) 5500 an ultrasound imaging system manufactured by and commercially available from Agilent Technologies, U.S.A., has a suitable bandwidth to achieve harmonic imaging with a single transducer thus providing a significant clinical improvement. Furthermore, the combination of the s4 transducer and the SONOS(trademark) 5500 provide multiple imaging parameter choices using a single transducer, thus providing a penetration choice as well as a resolution choice.
Conventional ultrasound scanners can create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the received ultrasonic echoes. In color flow imaging, the flow of blood or movement of tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissues or blood. The frequency of sound waves reflecting from the inside of blood vessels, heart cavities, etc. is shifted in proportion to the velocity of the blood cells. The frequency of ultrasonic waves reflected from cells moving towards the transducer is positively shifted. Conversely, the frequency of ultrasonic reflections from cells moving away from the transducer is negatively shifted. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In order to assist diagnosticians and operators the color flow image may be superimposed on the B-mode image.
Ultrasound images, like other images are subject to noise which may adversely affect the intensity values associated with the various pixels used to recreate the object or objects being observed. Ultrasound images, like some other images, also suffer from the effects of temporal noise in real-time image sequences. Conventional ultrasound imaging systems normally have an image frame filtering function, which acts on data in either polar or Cartesian coordinate formats.
One method for reducing temporal noise from an image is to use a filter which weights and sums corresponding pixel intensity values from the previous frame with a present input frame to generate a display pixel intensity. This is sometimes called xe2x80x9ctemporal filteringxe2x80x9d or xe2x80x9cpersistence filtering.xe2x80x9d In this method, a previous display frame""s pixel may be averaged with an input frame""s pixel, using a weighting value xcex1. The weighting value applies an equal degree of temporal filtering to all pixels in the frame. As a result, the method is data independent, i.e., not adaptive to changes in the underlying image data. While temporal noise is reduced, this simple filtering has the untoward effect of blurring or degrading small structures, the border of structures, or the borders of structures moving in the image field.
As will be further described below, continuous persistence filtering may be inappropriate when used in association with real-time imaging and high-power ultrasonic transmit pulses.
Harmonic imaging can also be particularly effective when used in conjunction with contrast agents. In contrast agent imaging, gas or fluid filled micro-sphere contrast agents known as microbubbles are typically injected into a medium, normally the bloodstream. Because of their strong nonlinear response characteristics when insonified at particular frequencies, contrast agent resonation can be easily detected by an ultrasound transducer. By using harmonic imaging after introducing contrast agents, medical personnel can significantly enhance imaging capability for diagnosing the health of blood-filled tissues and blood flow dynamics within a patient""s circulatory system. For example, contrast agent harmonic imaging is especially effective in detecting myocardial boundaries, assessing microvascular blood flow, and detecting myocardial perfusion.
The power or mechanical index of the incident ultrasonic pressure wave directly affects the contrast agent acoustical response. At lower powers, microbubbles formed by encapsulating one or more gaseous contrast agents with a material forming a shell thereon resonate and emit harmonics of the transmitted frequency. The magnitude of these microbubble harmonics depends on the magnitude of the excitation signal pulse. At higher acoustical powers, microbubbles rupture and emit strong broadband signals. In order to take advantage of these strong backscattered signals for imaging purposes, it is desired to have uniform destruction within the imaging plane. In general, the higher the transmitted frequency, the greater the variation in the response from the microbubble within the imaging plane. It has been determined that lower frequencies are more efficient at bubble destruction than higher frequencies.
Today""s systems, in order to deal with the lack of uniformity in bubble detection and bubble destruction, use multi-pulse techniques to increase the signal-to-noise ratio and to increase destruction of microbubbles. However, multi-pulse techniques for detection of contrast agents require the user to be able to discriminate motion artifacts from true bubble resonance and destruction signals. For example, in high mechanical index (MI) triggered techniques, such as Harmonic Power Doppler, it is critical that triggering occurs during that portion of the cardiac cycle when the heart is relatively stationary. Discriminating between motion artifacts resulting from triggering and variations in the cardiac cycle has made diagnosis difficult for users and has slowed acceptance of contrast imaging in clinical practices.
U.S. Pat. No. 5,410,516 to Uhlendorf et al. discloses contrast agent imaging along with single pulse excitation techniques such as harmonic imaging. Specifically, Uhlendorf teaches that by choosing a radio frequency (RF) filter to selectively observe any integer harmonic (2nd, 3rd, etc.), subharmonic (e.g., 1/2 harmonic) or ultraharmonic (e.g., 3/2 harmonic) it is possible to improve the microbubble to tissue ratio. The second harmonic has proven most useful due to the large bubble response at this frequency as compared to higher order integer harmonics, subharmonics or ultraharmonics. The second harmonic also is most practical due to bandwidth limitations on the transducer (i.e.,  less than 70% bandwidth, where percent bandwidth is defined as the difference of the high corner frequency xe2x88x926 dB point from the low corner frequency xe2x88x926 dB point, divided by the center frequency.) However, single pulse excitation techniques together with harmonic imaging suffer from poor microbubble-to-tissue ratios as large tissue integer-harmonic signals mask the signal generated by the contrast agent.
Currently in the field of ultrasound contrast exams, it is customary to use a low-acoustic power imaging technique to image ultrasound contrast agents in real-time. While this imaging is performed, a sequence of several high-power transmit pulses are directed into the tissue of interest for the purpose of destroying or modifying the contrast agent in the field of view. Under appropriate conditions, contrast agent backscatterers may be substantially removed from a region of interest. This methodology permits an operator to observe and record the re-perfusion of contrast agent within various tissues of interest.
Before the advent of contrast imaging and as described previously, the technique of applying persistence or temporal filtering to video images has been used to improve the appearance of the image by reducing the effects of thermal noise by averaging. However, when applying the persistence filtering technique to a contrast agent enhanced ultrasonic image where a destructive transmit sequence is applied, the brighter frames of the destructive sequence are averaged by the persistence technique, thereby obscuring several of the resulting image frames containing the re-perfusion of the tissue under observation.
In response to these and other shortcomings of the prior art, the present invention relates to an improved ultrasonic imaging system and method for harmonic imaging of contrast agent perfused tissues. Briefly described, in architecture, the system can be implemented with a transducer, an ultrasonic imaging system, a video processor having a persistence module, a patient interface, and a diagnostics processor.
The present invention can also be viewed as providing a method for synchronized persistence with contrast agent destruction and re-perfusion imaging. In this regard, the method can be broadly summarized by the following steps: introducing at least one contrast agent; insonifying tissue with an ultrasound signal suited to permit real-time observation of an organ of interest; modifying one or more parameters associated with the transmit signal to generate a contrast agent destruction sequence; synchronizing the persistence circuits in the ultrasound system with the destruction sequence so that image frames during the destruction sequence do not contribute to persistence filtered results. This allows the persistence filtering to be applied during the subsequent re-perfusion of the contrast agent in the tissues of interest without the adverse residual affects from a high-power ultrasonic reflection during contrast agent destruction.
Other systems, methods, features, and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.